Title

Confined Ferroelectric Properties in Poly(vinylidene Fluoride) PVDF-Based Random Copolymers and Graft Copolymers for Electric Energy Storage Applications

Date of Completion

January 2010

Keywords

Chemistry, Polymer|Engineering, Materials Science

Degree

Ph.D.

Abstract

Dielectric polymer film capacitors with high energy density, low loss, and fast discharge speed are highly desirable for compact and reliable electrical power systems. However, achieving both high energy density and low loss as well as fast discharge speed is difficult. Even though PVDF-based ferroelectric homopolymer and copolymers receive much attention due to the high dielectric constant and breakdown strength, the challenge for their application in electric energy storage lies in reducing the conduction and ferroelectric loss. The key is to understand the fundamental dipole reorientation and switching mechanisms in response to an oscillating electric field and in turn their effects on energy storage behavior. In this work, the compensation polarization in ferroelectric polymers was reduced by confining ferroelectric domains can facilitate a fast dipole reorientation during discharge and in turn the ferroelectric loss will be reduced and the discharged energy density will increase. ^ Based on the fundamental understanding, confined ferroelectric properties were achieved by confining the PVDF lamellar crystals with a low polarizability polystyrene (PS) interphase layer to reduce the compensation polarization in PVDF copolymers. Moreover, the confinement effect of different PS contents on ferroelectric and dielectric properties was investigated in a family of the P(VDF-CTFE)-g-PS graft copolymers with different PS grafting length. At low degree of confinement (i.e. low PS wt%), the antiferroelectric-like behavior was observed at an electric field as high as 400 MV/m as well as extremely low remnant polarization and low ferroelectric loss. At high degree of confinement (i.e. high PS wt%), the confined ferroelectric behavior had narrow hysteresis loops and became more like that for linear dielectrics. As a result, both dielectric and ferroelectric losses were significantly reduced at the expense of a moderate decrease in the discharged energy density. The best performance for high energy density and low loss dielectric capacitor applications was achieved in the P(VDF-CTFE)-g-PS with ∼34 wt.% PS, a relatively high discharged Ue of ∼10 J/cm 3 at 600 MV/m (extrapolated from 550 MV/m), a fairly low dielectric loss (tanδ = 0.006 at 1 kHz), and a low ferroelectric/conduction loss (undischarged% = 17.6% at 550 MV/m).^